I Mean, A lot Of Other Water-Logged Blocks (I.e slabs and stairs) Don't do this either. But those blocks are solid. You can walk right through a coral fan like its made of thin air. Blocks Like Ladders And Signs don't let water in due to functionality (Like Zombie Grinders With A Drop Down) But if I'm being honest these things would look sick if they were planted aside water for a  zombie grinder.